This invention relates to an image magnification control mechanism for a facsimile zoom lens.
A variety of methods have been provided for permitting picture size to be varied without increasing or decreasing the image characteristics, such as, e.g., resolution, in variable image magnification line scanning system facsimile apparatus of the type wherein optical data from different sized originals are converted into video signals by a line sensor, which video signals are then supplied to a transmission circuit. In one method wherein the magnification is varied with one fixed-focus lens, the distance between an object and its image is variable. Therefore, if the object side is fixed, then the photographing lens and the line sensor must be moved a substantial distance in order to properly form the image. Accordingly, the mechanism is necessarily intricate and is not practical.
An alternative method is one in which the focal length of a photographing lens is varied, so that magnification variation is carried out without changing the distance between an object and its image. Examples of this method are a turret system using a plurality of lenses in a switching mode and a zoom system. In the turret system, a plurality of fixed-focus lenses are selectively used by turning a lens holding plate. Therefore, if the distance between an object and its image is maintained unchanged for all of the lenses, it is difficult to provide a desired magnification. In the zoom system, one zoom lens is used to continuously change the focal length without changing the distance between an object and its image, thereby changing the picture size. Therefore, if the zooming position can be controlled accurately, then a desired magnification can be readily obtained.
This invention is intended to provide a mechanism for controlling a facsimile zoom lens to achieve image magnification variation for a line scanning system facsimile in which a zoom lens is used to change the image magnification.
The optical system of a facsimile zoom lens is made up of at least two groups of lenses having negative and positive powers, so that the image magnification can be varied without changing the distance between an object and its image. In general, the drive member of a zoom lens barrel for moving the above-described optical system comprises: a drive ring rotatable around the optical axis and having cam grooves defining a zoom region in the outer wall; an outer cylinder which is slidably fitted in the drive ring and in which lens group moving rings are slidably fitted, the outer cylinder having straight grooves or cam grooves to control the movements, in the direction of rotation, of the lens group moving rings; and lens moving rings in which roller pins are embedded, the roller pins sliding while engaging with the cam grooves and the straight grooves or the cam grooves in the outer cylinder.
The zooming is achieved as follows: As the drive ring is turned, the lens group moving rings are turned through the engagement of the roller pins with the straight grooves or the cam grooves of the outer cylinder, while the lens group moving rings are moved back and forth through the engagement of the roller pins with the cam grooves of the drive ring, so that the lens groups are displaced. Therefore, the image magnification of the zoom lens can be controlled by a method in which a position detector is provided for each lens group to control the position thereof. However, this method is not practical in that, if a plurality of lens groups are employed, then the positions of these plural lens groups must be controlled simultaneously and accordingly the construction is necessarily intricate. On the other hand, because of the facsimile's function, an image magnification must be quickly and accurately changed when a picture size is changed.